Now let me layout my project, I really need help thinking this through.

In summary we are going solar and will be replacing our existing 120,000 btu oil burner with a electric boiler system.

We are also "thinking" of installing a 3 ton air source heat pump in our downstairs living room/kitchen/dining area (one big room 40' x 30') with open stairway to second floor.

Regardless of rather we do the heat pump or not, we definitely want to get rid of the oil burner.

Currently we have old 3/4" iron pipe baseboard in series around the downstairs and upper level with two 24" x 30" cast iron radiators embedded in the wall between the living room area and a 20' x 30' garden room that also has radiant heat in it's tiled floor. The radiators are open to both the living area and the garden room but enclosed on the top and sides.

Now this is where it gets dangerous " I am thinking "
about replacing the oil burner/boiler with an electric boiler. Is anyone familiar with electric boilers? Any suggestions/warnings?

I want to split up the heating into three zones so I'll be looking for help in designing hook up for that but wanted to see what you had to say about the electric boiler and if you had any suggestions.

Oh I formally had a plumbing and heating business so I will be doing this work my self.

Round numbers...to replace a 120,000 BTU boiler you need about 40 KW. Solar does about 10 watts per square foot, so you would need 4000 sq feet of collectors just for that. And since much of your heating demand is in the nightime, what now kemosabe??

First, most houses on L.I. have design condition (= ~ +15F for most L.I. locations) heat loads well under 50K, so replacing a 120K (100K out) oil burner with a similarly oversized electric boiler is just ridiculous.

Second, ductless air source heat pumps (mini-split/multi-split) will have an average coefficient of performance of ~2.8-3.0 in a Long Island climate, reducing the amount of power used for space heating by about 2/3 relative to that of an electric boiler. The cost of the heat pump is a tiny fraction of the cost of the photovoltaics required to make up the difference in net-metered grid-power use.

A 3 ton multi-split can probably handle the whole-house load (or could with only modest insulation, window, & air-sealing improvements.) Heating with ductless costs about 1/3 that of heating with oil, even at L.I. electricity prices. (At this past year's oil pricing heating with an electric boiler would have be comparable to heating with oil.)

But any good heating system design starts with a carefully calculated room-by-room "manual-J" or similar heat load calculation. Knowing the room-by-room radiation and heat load at design temp would determine the water temps required to heat solely with the hydronic system. I'd be a bit shocked if it needed more than 140F, even if the existing system had been running with 160-180F water out of the oil boiler (necessary to keep the return temp to the boiler high enough to avoid destructive condensation.) If you have a winter oil bill with a K-factor stamped on it it's pretty easy to put a realistic upper bound estimate on the whole-house load at the outdoor design temp, but that won't address the room-to-room heat load differences. (But if you have that info, let's have it! And your zip-code too, to zoom in on actual heating design temps.)

Even if you used the electric boiler/hydronic system as the Hail-Mary backup for a ductless heat pump system, it would be less than half the power use of heating solely with an electric boiler. Most electric boilers are fully modulating systems and are easily micro-zoned, so if you re-plumbed the hydronic baseboards into a zone per room you could use the hydronic system to temperature-balance the place when heating the major spaces with ductless heat pump heads.